Conventionally, a known thermal-type airflow measuring device is employed for measuring a flow rate of air by utilizing heat transfer with air. Such a conventional airflow measuring device is, for example, equipped to a duct, which forms an air intake passage of an internal combustion engine, for measuring a flow rate (intake air amount) of intake air drawn into the internal combustion engine.
Such an airflow measuring device includes a case and a sensor. The case is equipped to a duct such that the case is projected into the duct for receiving a part of intake air flowing though the duct. The sensor is accommodated in the case and is configured to conduct heat transfer with the intake air received in the case and to send an electric signal correspondingly to the intake air amount in the duct. For example, JP-A-2008-309614, which corresponds to Publication of US Patent Application No. 20080307867, discloses an airflow measuring device having a configuration to protect the sensor from damage caused by collision with dust entering into the case.
Specifically, the case has a first passage and a second passage. The first passage draws intake air from the duct and exhausts the drawn intake air into the duct. The second passage branches from the first passage. The second passage draws intake air from the first passage and exhausts the drawn intake air into the duct separately from the first passage. The sensor is accommodated in the second passage. The first passage has a throttle at the downstream of a branch position where the second passage is branched from the first passage. The first passage exhausts intake air into the duct, while throttling the intake air at the throttle. The present configuration enables to apply inertial force on dust, which enters into the first passage, to move the dust toward the throttle thereby and to cause the dust to collide against the wall surface (throttle wall surface.) of the throttle to return the dust into the duct. Thus, the dust is restricted from entering the second passage.
Furthermore, in the airflow measuring device of JP-A-2008-309614, the throttle wall surface is formed such that dust, which once moves into the throttle, collides against the throttle wall surface thereby to be restricted from moving, i.e., reentering backward into the second passage.
In the airflow measuring device of JP-A-2008-309614, the throttle wall surface is formed on the assumption that collision of dust is an ideal collision described as follows. As shown in FIG. 11A, the ideal collision is a collision mode in which the incidence angle of a collision object relative to a collision surface substantially coincides with the reflection angle of the collision object relative to the collision surface. It is considered that, as the shape of a collision object becomes, for example, spherical, the collision mode becomes the ideal collision.
However, dust is in various shapes and is not necessarily in a spherical shape. Therefore, the collision mode of dust relative to the throttle wall surface is not necessarily the ideal collision. As shown in FIG. 11B, the collision mode of dust may become nonideal collision in which the incidence angle does not substantially coincide with the reflection angle. Therefore, even in a configuration in which the throttle wall surface is formed to restrain reentrance of dust on the assumption of the ideal collision, the reentrance of dust may not be effectively restrained. Thus, in order to protect the sensor from damage due to collision with dust, it is necessary to form the throttle wall surface in consideration of the nonideal collision.